Typical laboratory power supplies use local voltage sensing, in which the output voltage is regulated at the output terminals on the power supply. A laboratory power supply that uses local sensing is convenient to use, but the voltage across the load is not well regulated because of voltage drops in the power supply leads between the power supply output terminals and the load. The voltage drop depends on the resistance of the power supply leads and the current drawn by the load.
Precision laboratory power supplies may include a remote sensing feature that regulates the voltage at the load. In such supplies, sense terminals on the power supply are connected to the load separately from the current-carrying connections. Precision laboratory power supplies may also be set up for local sensing by connecting the sense terminals to the power supply output terminals. This type of power supply is inconvenient to use as a locally sensed power supply, because it is necessary to connect the sense terminals to the power supply output terminals. Some supplies include resistors connected between the sense terminals and the output terminals. Even when the resistors are present, the current through them is generally high enough to cause poor regulation at the power supply output terminals, if the sense terminals are not connected to the output terminals. Thus, the failure to connect the sense terminals to the output terminals in the locally sensed mode may result in poor regulation and unpredictable behavior.
Another approach to local sensing is disclosed by J. D. Felps in "Automatic Local Sensing Improves Regulation," EDN, Jan. 4, 1996, pages 102-104. A precise current is passed through the resistors that interconnect the sense terminals and the supply output terminals. Thus, the voltage at the power supply output terminals increases by a fixed amount, such as for example 2%, when remote sensing is not utilized. The voltage increase may compensate for the voltage drop through the leads that carry current to the load. This approach permits use of one power supply in multiple products, but is not particularly useful in laboratory power supplies. Accordingly, there is a need for improved sensing circuitry for controlling power supply voltages.